Multivibrator and control of same



April 4, 1950 J, wElNER 2,502,687

MULTIVIBRATOR AND CONTROL OF SAME 5 Sheets-Sheet 1 Filed D66. 30, 1944 amass/6. WE/NEP April 4, 1950 J R. WEINER 2,502,687

MULTIVIBRATOR AND CONTROL OF SAME Filed Dec. 30, 1944 3 Sheets-Sheet 2 Fwd AAAAAAAAA vvvvvvv IIIIIVV lllllll IVVII'I JV VC. 1 01.956

LLIJU L GHANA/4 I I I I l I I I I l I I l I OUTPUT 'HJIL INVENTOR. Jaws 6. Wf/A/EP Patented Apr. 4, .1950

MULTIVIBRATOR AND CONTROL OF SAME James R. Weiner, Brooklyn, N. Y., assignor to Radio Corporation of A of Delaware merica, a corporation Application December 30, 1944, Serial No. 570,665

12 Claims.

This application concerns improved voltage generators of the multivibrator type and novel application of the same to use in the radio and associated arts.

The general objects of my invention are improved multivibrators and improved wave generation, and improved use of the same in the signalling art.

- In known multivibrators the cross-coupled capacities charge through the tube grid to cathode impedances, and since these impedances are low during the charging time the grids are at or above ground potential and draw grid current, and the tubes of the multivibrator cannot operate as amplifiers.

There is considerable need of multivibrators which can be used as amplifiers in the radio art, as will appear hereinafter, and an object of my invention is to improve multivibrators in this respect.

An additional object of my invention is to provide a nultivibrator Which operates efficiently and is capable of wider use in the radio and allied arts than multivibrators known heretofore.

In multivibrator circuits known heretofore the grid is zero or positive when current flows through the tube. They are not to be used as amplifiers. grids of the tubes operate at negative potentials with respect to the cathodes, and the multivibrator may be put to a greater number of uses in the radio and allied arts. For example, either one or both sections of the multivibrator may be used as amplifiers alternately effective to amplify voltage applied to one or both of the tubes.

An object of my invention then is an improved multivibrator circuit wherein the tubes amplify voltages applied thereto to provide an output comprising oscillations of say different .frequencies alternately keyed at the fixed rate of vibration. Such a device is of considerable use as test equipment.

Another object of my invention is an improved electronic signal combining circuit making use of my improved multivibrator system. In this embodiment signals from different channels are impressed on the multivibrators and the same are controlled by synchronizing pulses to supply a combined output wherein the signals are combined as desired.

In a further'embodiment two systems as described in the preceding paragraph feed their outputs to the two sections of a further multi-- vibrator,'thus providing an electronic means for In my improved multivibrator the combining two channels or four channels as desired. Such means are entirely new in the multiplex and diplex art where heretofore mechanical arrangements have been used.

The above objects and others are attained by providing a charging path for each of the cross coupled condensers other than the grid tooath ode impedance of a tube. Means is then pro vided to block the charging paths through the grid to cathode impedances. This provides a multivibrator each section of which may operate as a class A amplifier because the control grids may now operate at a negative potential; In practice the charging paths may include diodes properly oled, and the blocking means may also include diodes properly poled. The cathode return circuits may now include resist-' ance, raising the cathodes positive with respect to ground and the grids, and current or potential may be applied to the cathodes for alternate amplification at the multivibrator rate of operation. lhe currents or potentials may be replaced by telegraphy signals on different chana nels and synchronizing impulses may be im{ pressed on the grids of the multivibrators and properly related as to time with respect to the signals to produce a combined output as desired.-

In the detailed description of my system which .follows, reference will be made to the attached drawings wherein Fig. 1 illustrates a multivibrator known in the This figure is used to make clear the art in which my invention resides and the manner;

art.

in which the improvement is obtained.

Fig. 2 illustrates an improved multivibrator containing additional charge paths arranged accordance with my invention to let the grids operate at negative potentials.

Fig. 3 is a modification of the arrangement of Fig. 2, and includes impedances in the oath-' ode return circuits for increasing the relatively negative potential on the tube grids so that the same may be used as amplifiers.

Fig. 4 illustrates an improved circuit arrange-j ment wherein oscillations of different frequencies: are amplified and supplied alternatively as out-f put from the multivibrator.

Fig. 5 illustrates a circuit arrangement wherein the multivibrato-r is used in a novel manner for; electronically combining signals from several channels.

ment of Fig. 5.

In Fig. b, to simplify the showing, the signals on the two channels are assumed to be made up of alternate marks and spaces at elemental baud rate.

Fig. 6 is a modification of the arrangement in Fig. 5. In Fig. 6 two systems, as illustrated in Fig. 5, are used to combine signals from two channels and the output from the multivibrators are then supplied as input to an additional electronic multivibrator so that we have as a final output signals combined from four channels.

In the usual form of inultivibratorshown in Fig. 1, the coupling capacitors Chl between the anode l3 and grid 1, and condenser Ch2 between anode 6 and grid M of the tubes 5 and I2, charge to the potential, +Ebb, through the grid to cathode resistances of tubes 5 and 12 respectively. Thus the grids "I and [4 are essentially at cathode potential, since the grid to cathode resistance RGK of each tube is extremely low, and the tubes 5 and 12 cannot be operated asclass A amplifiers. In addition, the points A and B tend to be positive with respect to ground and if a few volts of negative bias were appliedto'the grids they would remain at zero bias. Thus if a small signal were applied between the grids and cathodes, say between the cathode of tube 5 and ground through a resistor, no trace of the signal would be found in the voltage across RLI in the anode circuit because the grid would remain at or above zero bias throughout the signal variation.

Thus the use of multivibrators as known heretofore in the prior art is limited to the production of oscillations of the rate of vibration.

Inaccordance with one feature of my invention, as illustrated in Fig. 2, condenser charging paths other than the paths between the grids and cathodes of the tubes are :provided, i. e., the grid to cathode impedances RGKI and RGK2 of the tubes 5 and [2 are not used in my multivibrator.

Unilateral devices D2 and D4 are added as charging paths and these unilateral devices which may be in the form of diodes provide low charging resistors for condensers Chi and 0712 respectively. In order to prevent .charging of the condensers through the grid to cathode impedances RGKI and RGK2, unilateral devices DI and D3 are provided in the cross-couplings between the anodes andgrids of the tubes. These unilateral devices may also be diodes and are so represented. Thus, considering one section of .the multivibrator, Ch2 charges through the resistance of D4.and BM in series, and discharges through R92 and the resistance of diode D3 in serieswith the parallel combination of BL! and the resistance Rbl, the D. C. plate resistance of tube 5. The other section of the multivibrator including condenser .Chl charges through D2. in series withRLZ and discharges through Rol and DI in series with the parallel combination of RLZ and Rbi, the D. C. plate resistance of tube l2. .The grid voltage of tube i2 is now independent of the charging of GM to the potential Ebb since D3 provides an open cirouit in one direction. The grid to cathode voltage of tubes 5 and I2 will now depend upon a source of bias. This bias may be supplied in various manners. For example, the biasmay be supplied by the cathode return resistorsRKl and RK2 in the cathode circuits of the tubes 5 and I2, as shown in Fig. 3. The bias on the grid 1 is now equal substantially to the potential drop across RK! and this bias may be varied by varying the bias across RKI. The bias on the grid I4 is now substantially equal to the voltage drop across RK2 and this biasmay be varied by varying RK2.

" the natural period of the multivibrator.

Thus tubes 5 and I2 may be operated, for exanv ple, as class A amplifiers.

The operation of the arrangements of Figs. 2 and 3 are substantially the same, and the following description applies to both thereof. Resistors RKI and RKZ serve, as stated above, to raise the cathodes above ground potential. These resistors are not present in the arrangement of Fig. 2. Resistors RLI, RLZ, Rbl, Rb2, in conjunction with resistors Rgl and Rg2 and capacitors CM .and Chl provide the usual time constant circuit when the capacitors are discharging. Diodes D2 and D4 are provided, as stated above, to allow capacitorsChl and 021.2 to charge to the potential Ebb very rapidly, when the tubes 5 and I2 respectively cease .toflc'onduct. Diodes DI and D3 prevent the .charge.,,currents from flowing through resistors Rgl and R'g2 respectively but allow the entire discharge currents to flow through them, thus driving grids I and I4 below cutoff on alter nateportions of the cycle. The. multivibra-tor thus operates to generateoscillations the .frequency of which depends upon the size of capacitors Chl and CM and the value of resistors Rgl, Rg2, etc.,.and the voltage Ebb. 1

As stated above, the multivibrator of the present invention may be put to .wide use in the radio art. For example, oscillations tobe amplified may be applied to one or both of themultivibrator tubes. If oscillations of different fre quencies are applied as illustrated in Fig. 4, one to each multivibrator tube, a keyed output which shifts from one frequency to the other frequency and back is obtained. The rate of shift isat Such a device is of considerable use as test equipment, since it has an output which shifts at a fixed rate from one frequency to a second frequency.

In the embodiment of Fig. 4, an oscillator 50 of a first frequency fl has its output coupled across cathode resistance RKI, while an-oscillator 60 operating at a second frequency f2 has its output coupled across cathode resistance RKZ. Low pass filters l8 and 23 consisting of resistors l9 and 2|, and capacitors 20 and 22, and resistors 24 and 25, and capacitors 26 and 21, respectively, prevent the application of the oscillations amplified in the respective vibrator tubes 5 and. I2 to be applied to the grids ofthe other tubes 12 and 5, which would tend to synchronize the mul-' tivibrator. In other words, these filters included in the cross-couplings prevent the oscillations from source Blamplified by the multivibratojr tube I2 from being impressed upon the grid 1 of tube 5, and prevent the oscillations from source 50 amplified in tube 5 from being impressed upon the grid [4 of tube I2. In this manner, synchronization of the multivibrators by oscillations from sources 50 and/or 68' is prevented. Resistors RM and RLZ are connected to the source Ebb through the common output resistor Hi. The output is taken from across the resistor Ill, 1. (2., between the anode end of resistance In and ground. I

The oscillators 50 and 60 function continuously, and in an embodiment which operated successfully developed oscillations of diiferent tone frequencies across resistors RKI and RK2. Since the resistors RKI and RKZ are not in the oscillator circuits, they do not affect the frequency or amplitude of oscillation noticeably. Assuming tube 5 has just started to conduct, capacitor Chl will start to charge to voltage Ebb and a positive pulse of voltage will appear across diode D2;

atoms? resistor Rgl, and hence the grid of tube 5 will not be driven positive and will not limit but will assume some bias as determined by resistor RKI in the cathode. Thus capacitor Chi charges to voltage Ebb without affecting the operation of tube 5. The tone voltage applied to resistor RKI wil be amplified and will appear across resistors RLl and ID. The portion developed across resistor I is useful output voltage. The D. C. voltage at point AI will be less than voltageEbb by the drop in resistor l0 due to the flow of plate current of tube 5. If the circuit components are so adjusted that the D. C. plate current of tube is equal to that of tube l2, the D. C. potential at point AI will not change during a complete multivibrator cycle, and hence the keying frequency will not be coupled into the output at 10.

While tube 5 is conducting, capacitor CM is discharging through filter l8, diode D3, resistor R92, in series with the combination of resistor RLI in parallel with the internal resistance of tube 5 and cathode resistor RKI which are in series. At the instant at which 5 becomes conducting, the voltage from anode 6 to ground drops below voltage Ebb. This drop or negative step of voltage is coupled into resistor R92 and appears on grid l4, and is of such a magnitude as to stop the flow of plate current in tube l2. Capacitor Chl will then charge through resistor RL2, filter 23 and diode D2 all in series. It does not charge through resistor Rgl and hence does not affect the bias on grid 1 of tube 5, this bias being determined by the drop in resistor RKl'. The A. C. voltage appearing across resistor RKI due to oscillator 50 will be amplified by tube 5 and will appear across resistor I0 as output voltage. When the voltage across resistor Rg2 returns to the cutoff value for tube 12, this tube will begin to conduct and the preceding cycle of operations will now hold for this section of the multivibrator.

Th oscillators at 50 and 60 may be of any desired type, as long as they provide oscillations of the desired frequency, of substantially constant frequency and of sufficient amplitude to excite the multivibrator tubes.

In .an embodiment which operated extremely well the oscillators were of the RC type, one operating at a frequency of 1200 cps, and the other at a frequency of 1500 cps. The tubes 5 and I2 were of 6SN7 type. The diodes were tubes of the type known as 6H6 wherein two diodes are included in a single envelope.

Resistors RLI, RL2, l9, 2|, 24 and 25 equal 20,000 ohms.

Resistor I0 equals 5000 ohms.

Resistances RKI and RKZ are 1000 ohms.

U The fixed portions of resistances R9! and R92 are equal to 1 megohm.

The variable portions of resistances Rgl and R92 are variable and equal to 1 megohm.

Condensers 20, 22, 26 and 21 equal .001 microfarad.

Condensers CM and Chl equal .01 microfarad.

The multivibrator operated at about 25 cycles per second to provide pulses of oscillations of 1200 cycles per second and 1500 cycles per second, recurring at a rate of 25 cycles per second.

It will be understood that other tubes may be used and that the other appropriate circuit element constants may be selected depending in part on the frequency of operation desired.

The multivibrator of the present invention is particularly applicable to combining channels in a diplex or multiplex system. Such use is illustrated in Fig. 5 of the'drawings. "In Fig. 5 theoutput of one channel is impressed on the resistance RKI, while the output of the other channel is impressed on the resistance RK2. Synchronizing pulses are applied to the grid 1 and in phase synchronizing pulses are applied to the grid M of tube l2. The multivibrator switches the output circuit from one input channel to the other. The combination or aggregate of the two channels appears at the output.

Synchronous signals from printers, facsimile machines or other devices are applied to RKI and RK2 as shown. Now when tube 5, for ex ample, is conducting, the variations in voltage across resistance RKI produced by the signal in channel #1, will be amplified by tube 5, and will appear across resistance I0 as output voltage. At the same time, since tube I2 is in the nonconducting state, the variations in voltage across resistance RK2 will not appear in the output. When the multivibrator flips, the opposite set of conditions will hold true. For good operation and simplified setup procedure, the charge resistances (D2 and D4) for the capacitors Chi and C712 should be much smaller than the discharge resistances DI, D3, Rgl, Rg2, etc. This is equivalent to Rgl R2 and R92 R i, where R2 is the resistance of diode D2vand R4 is the resistance of diode D4. Since in a common set up By! might be from K to 500K and R2 might be from 500 ohm to 1K this condition is certainly satisfied. The above values also hold, of course, for R92 and R4. When used in a diplex system the multivibrator will be symmetrical and if used with facsimile and Continental Morse channels, the multivibrator rate should be equal to the single channel baud rate. As used with facsimile machines the conditions in each channel and in the output are similar to those shown below.

In the absence of synchronizing pulses on the control grids 1 and M the multivibrator oscillates atits free frequency and signals from channels #1 and #2 feed through tubes 5 and I2 alternatively for like time periods to the output. With applied synchronizing pulses the time of conductivity of the multivibrator sections is controlled and the time relation of the synchronizing pulses on tubes 5 and I2 with respect to the signals is such that the middle half of signal elements of each channel is keyed through to provide an aggregate output. The channels are successively assigned to the output by the multivibrator sections and in the output the two channels are given. equal intervals of time, the intervals being equal to the duration of a signal element. The

duration of a signal element is the shortest time interval required for a mark or space signal, and all longer mark or space elements are integral multiples of the signal element.

The signal in each channel are so phased with respect to the synchronizing pulses that the multivibrator picks out the center of each signal element in each channel. Synchronizing pulses of twice the keying rate as shown on the first line, Fig. 5a, are applied to the grids 1 and M to control as desired the starting and stopping of said tubes. The signal on say channel #1 starts of a signal element ahead of the signal on channel #2. The synchronizing pulses applied to the grid i of tube 5 amplifying the output of channel #1 are applied in such time relation with re' spect to the signal pulses on channel #1 that the start of the conduction time of tube5 lags be- On application of the "synchronizing" pulse tube 5 conducts and tube [2 is out off. Tube 5 conducts for A of a signal element, represented at a in line I of Fig. 5a. The synchronizing pulse on the grid M of tube I2 turns this tube'on. The signal element on channel #2 lags the time at which tube 5 was turned on by of a signal element so that tube [2 begins to conduct & of a signal element aiter tube 5 started to conduct. When tube l2 starts to conduct tube 5 is cut oif. Tube l2 conducts for of a signal element, represented at b in line i of Fig. 5a, and then a synchronizing pulse turns tube 5 on and due to the multivibrator action tube I2 is cut off. Tube 5 is conductive during of the signal element on channel #1 while tube l2 is conductive during of the signal element on channel #2. Then the output due to the signal in channels #1 and #2 represents the cen ter portion of each signal element on channels #1 and #2.

. In Fig. 5a signal elements as they may occur in practice are shown on channels #1 and #2. In orderv to simplify the showing the signals on channels #1 and #2 are in Fig. 5b assumed to be made up of alternate marks and spaces at elemental baud rate. To repeat the above statement of operation, the signal in channel #1 is applied to grid 7 A; of a signal element ahead of the synchronizing pulse (line 2).. Tube 5 is as- 'sumed to be cut off and tube [2 on. An applied synchronizing pulse on the grid l turns tube 5 on; current flows and turns tube l2 off. The bias on grid I2.is becoming less negative and the adjustment is such that the synchronizing pulse applied to grid M of tube l2 of a signal element later raises this grid of tube l2 above cut off and tube l2 is turned on, current starts to flow and tube 5 is turned Oh. The operation is then repeated.

The mannerin which the channels are combined is essentially as disclosed by Callahan, Mathes and Kahn in an article. entitled Time division multiplex in radio telegraphic practice, appearing in the IRE ofJanuary 1938.

I have disclosed an improved and emcient means for combining two channels in a diplex or multiplex system. in Fig. 5 of the drawings. Heretofore combining channels in multiplex or diplex systems has been carried out by mechanical means instead of electronic means as in my improved system. .In certain systems known heretofore combining units have consisted of a commutator with a number of segments, at least one segment being allotted to each signal channel, and with a brush arm which moves over the segments sequentially. This mechanical combining means requires a motor or other driving means, a shaft and all of the equipment necessary to the use of such means. In my system, as stated above, the combining is accomplished electronically by an improved multivibpator operated in an improved manner.

The principle involved in my improved elec tronic signal combiner may be extended as illustrated in Fig. 6 to a four-channel time division multiplex system. These multivibrators MW and MVZ are each as illustrated in Fig. 5 and described hereinbefore. Synchronizing pulses and signals from channels #1 and #2 are applied to MVl to provide at its output an aggregate signal as described above. Synchronizing pulses signals from channels #3 and #4 are applied to MV2 in similar manner to provide at its output an aggregate signal. The operation of each multivlbrator to select and supply as output the center half of each signal element on each channel is as described above in connection withFig. .5. The outputs of MVI and MVZ are then applied as channels #1 and #2 to the respective sections of a third combiner multivibrator MV3 in which the grids are controlled by synchronizing pulses to supply at the output of MV3 the aggregate of channels #1, #2, #3 and #4. The multivibrator MV3 is operated to select the center halves of the signal elements on each channel just as described above in connection with Fig. 5. The signal on channel #1 represents the signal on channels #1 and #2, While the signal on channel #2 represents the signal on channels #3 and #4. At the output of MV3 the signals from channels #1 and #2 are combined.

What is claimed is:

1. In a wave generator of the multivi'brator type including two' electrode structures each in cluding an anode, a cathode and a control electrode, a source of potential coupled to the anode and to the cathode of each electrode structure, a resistor in each anode coupling, a condenser and a unilaterally-conductive device connected in series between the anode and cathode of each electrode structure, said devices each being poled to permit charging of its corresponding seriesconnected condenser therethrough from said source, means connecting each of said condensers in series with a unilaterally-conductive device between the anode of one electrode structure and the control electrode of the other electrode structure and between the anode of the other elec trode structure and the control electrode of the one electrode structure, said last-named devices each being poled to permit discharge of its corresponding series-connected condenser therethrough. i

2. In a multiplex signalling system, at least two incoming channels each having pulsating signal intelligence thereon, at least one outgoing signalling channel, a multivibrator including two electrode structures each including an anode, a cathode and a control electrode, with the anodes maintained positive with respect to the cathodes and the anodes and control electrodes crosscoupled by capacitors, means for allowing the control electrodes to be biased negatively with respect to the corresponding cathodes so that the said electrode structures operate as amplifiers, connections from one incoming channel to one electrode structure for impressing the pulsating signal intelligence thereof on such electrode structure, connections from the other incoming channel to'the other electrode structure for impressing the pulsating signal intelligence thereof on such electrode structure, connections for impressing synchronizing impulses on each electrode structure, and means connecting both of said electrode structures to said outgoing channel.

3. In a signalling system, at least four incoming signal channels, at least one outgoing signal channel, three multivibrators each including two electrode structures, each structure having an anode, a cathode and a control grid, with the 6f the multivibrators, means for su'pplyingthe outputs of said one and another multivibrator as signal channels to the respective electrode structures of said third multivibrator, the signal elements of the respective channels being relatively time displaced a fraction of the time duration of a signal element, means for impressing synchronizing pulses on the electrode structures, and a coupling between the third multivibrator and said outgoing signal channel.

4. In a signalling system, in combination, a Wave generator of the multivibrator type comprising two electrode structures each having an anode, a cathode and a control grid, with the control grids and anodes cross-coupled by condensers and the anodes maintained at positive potentials relative to the cathodes, and the control grids maintained negative relative to the cathodes, thereby increasing the impedance between the control grids and cathodes, a charging path for each condenser including a diode in a connection between the terminal of each condenser connected to the control grid of an electrode structure and the cathode of said electrode structure, and means in the cross-coupling for preventing charging of the condenser through the grid to cathode impedances of electrode structures, a resistance coupling the cathode of each electrode structure to ground, two sources of potentials to be amplified, means for impressingpotentials from one source on one resistance, means for impressing potentials from the other source on the other resistance, and an output circuit coupled to the anodes of the electrode structures.

5. In a wave generator of the multivibrator type including two electrode structures each having an anode, a cathode, and a control grid, a source of potential coupled by a, resistance to the anode of each electrode structure, a condenser and a diode in series coupled between the anode and cathode of each electrode structure, crosscouplings including the said condensers in series each with a diode between the anode of one electrode structure and the grid of the other electrode structure and between the anode of the other electrode structure and the grid of the one electrode structure, a resistance coupling the cathode of each electrode structure to ground, an output circuit coupled to the anodes of the electrode structures, and input circuits coupled to the last mentioned resistances.

6. In a signalling system, in combination, a wave generator of the multivibrator type including two electrode structures each having an anode, a cathode and a control grid with the anodes maintained at potentials positive with respect to the cathodes and the control grids and cathodes cross-coupled by condensers, means for maintaining the control grids negative with respect to the cathodes, thereby increasing the impedances between the control grids and cathodes, and reducing their efiectiveness as charging paths for the condensers, a charging path for each of said condensers including a diode, and means in the cross-coupling for preventing charging of the condenser through the grid to cathode impedances of the electrode structures, a resistance coupling the cathode of each electrode structure to ground, two sources of signals formed by signal elements of like duration, the signal elements on the two channels being relatively displaced in time by a half of a signal element, means for impressing signals from one source on one resistance, means for impressing signals from the other source on'the other re'-' sistance, and means for impressing synchronized pulses on the grids of the electrode structures, the synchronizing pulses recurring at a rate double the frequency of the signal elements and lagging the signal elements on each channel in time by one quarter of a signal element.

7. In an oscillation generator of the multivibrator type, two electron discharge systems each having an electron source electrode, an

electron flow control electrode, and an electron receiving electrode, means for applying direct potentials to the electron receiving electrodes which are electro-positive with respect to the direct potentials on the electron source elec trodes and capacitors cross-coupling the electron flow control electrodes and electron receiving electrodes, said capacitors each having one terminal coupled to the electron receiving elec-t trode of one system and another terminal ecupled to the electron flow-control electrode of the other system, a direct current charging path for control electrodes and the capacitors and poled oppositely with respect to the first-named devices,

for preventing charging of said capacitorsthrough the impedances between the electron source electrodes and electron flow control elec' trodes of said systems.

8. In an oscillation generator of the multivibrator type, two electron discharge systems each having a cathode, a control electrode, and

an anode, a resistor for each system connected to the anode thereof for applying a direct potential to the anode which is positive with re' spect to the direct potential on the cathode thereof and capacitors cross-coupling the control electrodes and anodes, said capacitors each.

having one terminal coupled to the anode of one system and another terminal coupled to the control electrode of the other system, a biasing circuit connecting the control electrode of each system to its cathode for operating the control electrodes negative relative to the cathodes, each of said biasing circuits comprising a resistor connecting its tubes cathode to ground and a second resistor connecting its tubes grid to ground said negative biases reducing the effectiveness of the grid to cathode impedances of said systems as charging paths for said capacitors, and a separate charging path for each of said capacitors, each charging path including in series a different one of said first-mentioned resistors and a unilaterally-conductive device connected between ground and said other terminal of each of said capacitors and poled to pass current to ground.

9. In an oscillation generator of the multivibrator type, two electron discharge systems each having a cathode, a control electrode and an anode, a resistor for each system having one end thereof connected to the anode of its system and its other end coupled to a source of direct potential which is positive with respect to the potential on the cathode of its system, capacitors cross-coupling the anodes and control electrodes of said systems, a resistor connecting the cathode of each system to ground to bias such cathode, a direct current charging path for each of said capacitors including in series therewith and with oneof said first-mentioned resistors a unilaterally-conductive device having one terminal thereof connected to ground and poled to pass current to ground, and unilaterallyconductive devices, one in each of the crosscouplings between the control electrodes and the capacitors and poled oppositely with respect to the first-named devices, for preventing charging of the capacitors through the impedances be tween the control electrodes and cathodes of said systems.

10. In a signalling system, in combination, a wave generator of the multivibrator type includ ing two electron discharge systems each having a cathode, a control electrodeand an anode, means for applying to the anodes direct potentials which are positive with respect to the potentials on the cathodes and a capacitor and diode in series cross-coupling the anode of each discharge system to the control electrode of the other system, said capacitors each having one terminal coupled to the anode of one system and another terminal coupled by a corresponding diode to the control electrode of the other system, said diodes each being poled to prevent current flow therethrough to the coupled control electrode, a direct current charging path for each of said capacitors, including a separate unilaterally-conductive device connecting said other terminal of each capacitor to a point of negative potential, said devices being poled to pass current to said point, and means for applying varying potentials to be amplified to each of said discharge systems.

11. In a signalling system, in combination, a wave generator of the multivibrator type including two electron discharge systems each having a cathode, a control electrode and an anode, means for applying to the anodes direct potentials which are positive with respect to the potentials on the cathodes and capacitors crosscoupling the anodes and control electrodes, said capacitors each having one terminal coupled to the anode of one system and another terminal coupled to the control electrode of the other system, a biasing circuit connecting the control electrode of each system toits cathode for operating the control electrodes negatively relative to the cathodes, the negative biases increasing the effectiveness of said systems as amplifiers by increasing the impedances between the control electrodes and cathodes, a, direct current charging path for each of said capacitors, including a separate unilaterally-conductive device connecting said other terminal of each capacitor to a point of negative potential, said devices being poled to pass current to said point, and separate sources of oscillations of difierent frequencies coupled one to the cathode of each discharge system.

12. In a signalling generator, two sources of oscillations of different frequencies, a multivibrator including two electron discharge systems having their control electrodes and anodes cross-coupled and supplied by direct potentials such that systems are alternatively operative as amplifiers, a capacitor and a diode in series in each of said cross-couplings, the diodes being poled to prevent current flow therethrough to the coupled control electrode, and a charging path for said capacitors including a diode connect-" ing the control electrode end of each capacitor to the cathodes of said systems, said last-named diodes being poled to allow current flow there through toward said cathodes, connections from one source to one system for impressing oscillations from one source on one system for amplification, connections from the other source to the other system for impressing oscillations fromthe other source on the other system for amplification, and an output circuit coupled to the two systems.

JAMES R. WEINER,

REFERENCES CITED The following references are of record in the" file of this patent:

UNITED STATES PATENTS Number Name Date 2,048,081 Riggs July 21, 1936 2,086,351 Callahan July 6, 1937 2,159,792 Geiger May 23, 1939 2,189,317 Koch Feb. 6, 1940 2,182,555 Geiger Dec. 5, 1939 2,262,838 Deloraine et a1. Nov. 18, 1941 2,207,511 Geiger July 9, 1940 2,272,070 Reeves Feb. 3, 1942 2,282,895 Shepard May 12, 1942 2,300,999 Williams Nov. 3, 1942 2,380,520 'Hassler July 31, 1945 2,419,292 Shepard, Jr Apr. 22, 1947 

